Introduce NullIndicators and a Config option for it

Mostly for debugging purposes, but also useful if anyone doesn't
want to display indicators

src/private/DropIndicatorOverlayInterface_p.h

@@ -81,6 +81,7 @@ protected:
     DropArea *const m_dropArea;
     bool m_draggedWindowIsHovering = false;
 };
+
 }
 
 #endif